The present invention relates to new polymeric compositions and their application in the production of semi-permeable membranes.
The use of semi-permeable membranes in separation processes such as reverse osmosis and ultrafiltration is known. For these applications it is desirable to produce a screen-supported or reinforced membrane, that is, a membrane in which is embedded a flexible support, which is often a woven fabric, non-woven fabric or a net, which support makes the membrane at the same time strong and easily manipulable, even with large-sized membranes. The production of screen-reinforced membranes is generally carried out by casting a solution of polymer on the reinforcement followed by removal of the solvent (particularly by evaporation and/or coagulation). The polymer solution is hereafter referred to as the "collodion", regardless of the nature of the polymer. When excessively dilute collodions are used, it is difficult to obtain a uniform coating, and if the cast collodion is thick, the collodion tends to spread out and overflow, and this leads to an irregular thickness. If the cast collodion is thin, the screen reinforcement may not be entirely covered; in addition, when the membrane is coagulated after the collodion has been cast, the inevitable ripples on the surface of the coagulating bath form surface irregularities (folds) on the membrane and, furthermore, the polymer (in a dilute collodion) coagulates incompletely and material breaks away therefrom and denatures the coagulant liquid by forming suspensions. It is therefore of very great interest, from an industrial point of view, to be able to use fairly concentrated solutions of the polymers.
Very many known polymers have been tried or used for making semi-permeable membranes. Polymers based on derivatives of polysulphones are interesting, essentially because of the chemical stability inherent in the very nature of polysulphones, even when they are made into membranes.
Membranes, made from polysulphones described in British Specification No. 1,238,180 are discussed in French Certificate of Addition No. 2,005,334. The production of these membranes is carried out using a collodion containing from 8 to 20% of polymer. If it is desired to make membranes from polysulphones using more concentrated solutions, for example containing at least 25%, such that the viscosity is greater than at least 15 poises, membranes suitable for ultrafiltration can no longer be obtained.
It has also been proposed to use sulphonated polysulphones (see for example, British Specifications No. 1,258,851 and No. 1,350,342). These polymers give interesting results, but one object of the invention is to improve them especially with respect to permeability.
Polymeric compositions derived from polymers of the polysulphone type have now been found, according to the present invention, which provide access to semi-permeable membranes which are industrially interesting and can be made from fairly concentrated collodions.